Bug 14201: Yes, goodbye to the third plugin marc21_leader_video too

Bug 14201: Yes, goodbye to the third plugin marc21_leader_video too

A closer look at this plugin revealed the same things as earlier:
[1] Since the click redirects to leader, the launcher is not used.
    In this case no template file existed.
[2] Leader_video does not use Focus while leader does.

This patch removes 'all' traces of leader_video.
It includes a tiny db revision for those brave people using it. The only
behavior change for them is the focus change.

Test plan:
[1] Run the db revision.
[2] Check if the leader plugin still works as expected.
[3] Git grep on marc21_leader_video. Ignore atomicupdate. No PO files :)

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>